Directly call gettid syscall instead of library function

Author: Azoy

stdlib/public/SwiftShims/swift/shims/SynchronizationShims.h

@@ -23,6 +23,10 @@
 
 #define SWIFT_FUTEX_WAITERS FUTEX_WAITERS
 
+static inline __swift_uint32_t _swift_stdlib_gettid() {
+  return syscall(SYS_gettid);
+}
+
 static inline __swift_bool _swift_stdlib_wait(__swift_uint32_t *lock) {
   return syscall(SYS_futex, lock, FUTEX_LOCK_PI_PRIVATE,
                  /* val */ 0, // this value is ignored by this futex op

stdlib/public/Synchronization/CMakeLists.txt

@@ -98,6 +98,8 @@ add_swift_target_library(swiftSynchronization ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES
     ${SWIFT_SYNCHRONIZATION_DARWIN_DEPENDENCIES}
   SWIFT_MODULE_DEPENDS_WATCHOS
     ${SWIFT_SYNCHRONIZATION_DARWIN_DEPENDENCIES}
+  SWIFT_MODULE_DEPENDS_WINDOWS
+    ${SWIFT_SYNCHRONIZATION_WINDOWS_DEPENDENCIES}
 
   SWIFT_COMPILE_FLAGS
     ${SWIFT_SYNCHRNOIZATION_SWIFT_FLAGS}

stdlib/public/Synchronization/Mutex/LinuxImpl.swift

@@ -12,10 +12,6 @@
 
 import SynchronizationShims
 
-@_alwaysEmitIntoClient
-@_extern(c, "gettid")
-func _gettid() -> UInt32
-
 extension Atomic where Value == UInt32 {
   borrowing func wait() {
     _swift_stdlib_wait(.init(rawAddress))
@@ -33,8 +29,8 @@ extension Atomic where Value == UInt32 {
 internal struct _MutexHandle: ~Copyable {
   // There are only 3 different values that storage can hold at a single time.
   // 0: unlocked
-  // _gettid: locked, current thread's id (uncontended)
-  // (_gettid | SWIFT_FUTEX_WAITERS): locked, current thread's id (contended)
+  // TID: locked, current thread's id (uncontended)
+  // (TID | SWIFT_FUTEX_WAITERS): locked, current thread's id (contended)
   @usableFromInline
   let storage: Atomic<UInt32>
 
@@ -49,7 +45,7 @@ internal struct _MutexHandle: ~Copyable {
   @usableFromInline
   borrowing func lock() {
     // TODO: Is it worth caching this value in TLS?
-    let selfId = _gettid()
+    let selfId = _swift_stdlib_gettid()
 
     // Note: We could probably merge this cas into a do/while style loop, but we
     // really want to perform the strong variant before attempting to do weak
@@ -97,7 +93,7 @@ internal struct _MutexHandle: ~Copyable {
   borrowing func tryLock() -> Bool {
     storage.compareExchange(
       expected: 0,
-      desired: _gettid(),
+      desired: _swift_stdlib_gettid(),
       successOrdering: .acquiring,
       failureOrdering: .relaxed
     ).exchanged
@@ -107,7 +103,7 @@ internal struct _MutexHandle: ~Copyable {
   @usableFromInline
   borrowing func unlock() {
     // TODO: Is it worth caching this value in TLS?
-    let selfId = _gettid()
+    let selfId = _swift_stdlib_gettid()
 
     // Attempt to release the lock. We can only atomically release the lock in
     // user-space when there are no other waiters. If there are waiters, the